1. Technical Field
The present invention relates to a brushless motor and, more particularly, to a motor for an electric power steering device which supports steering effort of a steering wheel of a vehicle.
2. Description of the Related Art
In recent years, electric power steering (EPS) system that supports steering operation by rotating a motor with a vehicle battery is employed. The EPS system is attracting attention as an efficient system with smaller power loss of an engine as compared with the case of generating an oil pressure by rotation of the engine. On a motor used for such an EPS system, a part as position detecting structure is mounted in order to realize high-precision control.
Since the rotary shaft of a motor used for the EPS system is connected to the driver indirectly via a steering wheel, cogging and torque ripple generated from the motor is directly transmitted as an abnormal state of the steering wheel to the driver. Consequently, it is necessary to reduce the cogging and the torque ripple. To solve the problem, a method of accurately disposing position detecting structure has been devised.
Next, conventional structures of the EPS will be described with reference to FIGS. 7 and 8.
FIG. 7 shows a first conventional structure in which a resolver 1 as position detecting structure is disposed on an axial outside of an inner space of ball bearings 2 axially apart from each other.
In the first conventional structure, however, since the resolver 1 is disposed on the axial outside of a space between a ball bearings 2 axially apart from each other, the space between the ball bearings 2 axially apart from each other has to be narrowed only by a space 3 in the axial direction in which the resolver 1 is disposed. As a result, when the distance between the ball bearings 2 is shortened, accuracy of a shaft 4 attached to the ball bearings 2 cannot be assured. Therefore, the shaft 4 swings and it may cause vibration of the motor. Moreover, in the first conventional structure, the resolver 1 is exposed to the axial outside, so that a circumferential position adjustment of the resolver 1 is performed after the motor is assembled. There is consequently the possibility that a member other than the motor comes into contact with the resolver 1 and the resolver 1 is damaged.
FIG. 8 shows a second conventional structure in which a board holder 5 including a rotation position detecting board 5b on which a hall device 5a as position detecting structure is mounted is disposed on an axial inside of the space between ball bearings 7 axially apart from each other. A sensor magnet 6 is axially disposed so as to face the rotation position detecting board 5b. The board holder 5 is disposed under a bracket 8 and is fixed to the bracket 8 by only three screws 9 screwed from an upper side of the bracket 8. Insertion holes 8a of the screws 9 in the bracket 8 are circumferentially formed in an arc shape. By moving the screws 9 among the insertion holes 8a in the circumferential direction, the circumferential position adjustment of the board holder 5 is performed.
In the second conventional structure, however, after the motor is assembled, there is no member supporting the board holder 5 below the board holder 5. Consequently, the board holder 5 cannot be fixed by being axially sandwiched. The board holder 5 cannot be fixed with reliability, and a low-reliability motor is produced in which the board holder 5 may come off due to motor vibrations, an external collision, or the like. Moreover, since the circumferential position adjustment of the board holder 5 is performed by moving the screw 9 in the circumferential direction, the board holder 5 has to be circumferentially moved by holding the head of the screw 9. It is difficult to move the board holder 5 by holding only the head of the screw 9 and a problem of low workability arises.